This invention relates to power supplies for certain lamps. More particularly, the invention may be used to prolong the useful life of lamps that exhibit a progressive degradation in light energy output in response to the application of electrical power, such as arc lamps.
A photo-initiated adhesive is responsive to light energy when in liquid form to begin setting into a solid form. The photo-initiated adhesive must be exposed to a pre-determined amount of light energy in order for the setting process to begin. Light energy of various wavelengths may be used, depending on the particular photo-initiated adhesive. In many cases, arc lamps are used to provide the required light energy.
A known problem with arc lamps is that they exhibit a progressive degradation in their light energy output during their useful service lives. An arc lamp typically has a sealed region containing a gas which may be under pressure and a pair of electrodes separated by a gap. The gas may comprise mercury, argon, zenon and other gases. A voltage potential is applied across the electrodes by a power supply, causing an arc to be generated in the gap. The arc causes the gas in the gap and the region adjacent to the gap to form a plasma, which emits light. The wavelength of the emitted light depends on the gas used.
The lamp usually has a reflector which focuses most of the emitted light into a light delivery device, such as a fiber optic light guide. The light delivery device is used to direct the light onto the photo-initiated adhesive. The reflector may be said to provide an optical coupling between the lamp and the light delivery device. The optical coupling is configured to direct a known amount of light into the light delivery device, when the lamp is in new condition.
Over time, the electrodes in the lamp deteriorate in response to the electrical power applied across them. Typically, the rate of this deterioration is higher when a larger power signal is applied to the lamp. As the electrodes deteriorate, the gap between them becomes larger. The larger gap length increases the impedance of the lamp, resulting in the lamp drawing less current from the power supply. Also, the larger gap length requires a larger voltage potential across the electrodes to maintain the arc. As the current drawn by the lamp falls, the power consumed by the lamp also falls. The degradation of the electrodes and the corresponding drop in power drawn by the lamp would normally reduce the light emitted by the lamp.
To prevent the light output of the lamp from falling, the lamp""s power supply is typically configured to apply a constant amount of power by increasing the voltage applied to the lamp during the lifetime of the lamp, as the current drawn by the lamp falls. The constant power input results in a relatively constant light output from the lamp.
The larger gap length also results in a longer arc and a correspondingly larger physical volume of plasma from which light energy is emitted. As the volume of plasma increases and more generally, as the shape of that volume changes, the optical coupling between the lamp and the light delivery device may become less efficient, resulting in less light being directed into the light delivery device, despite the fact that the light output from the lamp remains relatively constant. The reduced amount of light being coupled into the light delivery device directly reduces the amount of light energy that may be used to expose a photo-initiated adhesive.
The life of the electrodes may be extended by applying a smaller power signal across them. However, this solution has the disadvantage that the lamp will emit less light. Furthermore, the electrodes will still deteriorate, (although at a slower rate) and accordingly, the light coupled into and emitted from the light delivery device will still decline during the life of the lamp.
The effect of reduced light being coupled into the light delivery device during the service life of the lamp may be partially compensated for by increasing the power applied to the lamp during its life, rather than keeping the power level constant. As the lamp ages, the increased power input results in an increased light output from the lamp. The increased light output may compensate (at least in part) for the degraded optical coupling between the lamp and the light delivery device by slowing the absolute reduction in light energy focused into the light delivery device.
One power supply that implements this solution provides a power output adjustment for manually adjusting the power level applied to the lamp. In this system, a user initially sets the power level to a fraction of the rated power level of the lamp. Then as the lamp ages, the user periodically increases the power level applied to the lamp. Eventually, the lamp burns out or is replaced for another reason. The user must then reset the power level to the initial level for the new lamp. This manual system has several disadvantages. First, a user may forget or neglect to periodically increase the power applied to the lamp resulting in a low light output level from the lamp. Second, if the user has adjusted the power signal to a high level during the life of one lamp and then does not return it to its proper initial level when a new lamp is installed, the new lamp will deteriorate more rapidly. Third, a user will normally have no way of knowing what power level should be set for a previously used lamp which is being re-installed, unless the user recorded the setting when the lamp was removed. Fourth, the user may intentionally set the power signal to its highest level in order to increase the light output of the lamp. This will defeat the purpose of the adjustment and result in the lamp having a short useful service life. In combination, these disadvantages are likely to lead to the adjustment being ignored or used incorrectly.
Accordingly, there is a need for an improved power supply for use with lamps that exhibit a change in the size and shape of the arc resulting in a change in their light energy output in response to the application of a power signal. Preferably, the new power supply will automatically adjust the power supplied to the lamp. When used with an arc lamp, the improved power supply will preferably reduce the rate of deterioration of the electrodes of lamp, thereby reducing the rate at which the light energy output of the lamp degrades, as well as reducing the effects of light coupling degradation.
The present invention provides a power supply for a lamp which exhibits an erosion of its electrodes or of another component of the lamp in response to the application of an electrical potential to the lamp, resulting in a change in the power drawn by the lamp from the power supply. The power supply provides an output power signal with a current and a voltage component to the lamp. As the electrodes of the lamp deteriorate, the lamp""s resistance falls and it draws less current from the power supply. This fall in the current component of the output power supply is measured and is used to generate a skewing signal. The voltage across the lamp is controlled by the power supply and it is also measured. A control signal is generated based on the measured voltage and current components of the output power signal applied to the lamp. A feedback loop is used to regulate this control signal by increasing the magnitude of the voltage component of output power signal to (i) compensate for the fall in the current component of the output power signal and (ii) in response to the skewing signal, which amplifies the effect of the fall in the current component. The voltage component of the output power signal is thus increased more than is required to keep the power magnitude of the output power signal constant in response to the fall in its current component. The power magnitude thus rises in response to the fall in the current component, which is correlated to the deterioration of the lamps electrodes and the related change in the size and shape of the lamp""s light output.
The power supply of the present invention is configured to initially power the lamp at a portion of its rated power level. Typically, this portion will be between 50% and 90% of the rated power level. The portion may be less than 50%, although this may cause an undesirable loss in light output from the lamp. More preferably, the portion is between 70% and 85% of the rated power of the lamp. During the life of the lamp, the power signal is steadily increased in response to the reduction in current drawn by the lamp. Towards the end of the service life of the lamp, the power signal applied to the lamp may be close to the rated power of the lamp.